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```Adjustable 350T Regulator

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Adjustable 350T Regulator

Written By: Steven Robert Cypherd

SUMMARY

Anyone that has played with these little workhorses of our world and wondered how they
work, read on. The standard three-terminal regulators like the 7805 (5v) are what most
people see and use without thinking too much about them. All voltage regulators work about
the same. They read a reference voltage between Volts In and Ground and/or Volts Out and
they use this reference to set the output voltage. When a load is applied, this reference
keeps the output voltage steady.

You can play games with these old regulators by putting a resistor between the Ground Pin
and Ground. This can make a standard 7805 give any voltage between 5 volts and your
supply voltage. This shows how much regulators are the same.

About supply voltage to a regulator: You need to have at least the regulated voltage plus 2/3
more for basic operation of a regulator. That is, 5 plus about 4 equals about 9 volts, the
minimum to get a steady regulated output from a 7805. You also need the current capability
of your supply to equal at least twice as much current as your circuit needs. Why? Too little
input current equals poor regulation. The regulator will lose its reference voltage.

A good starting point is about twice the voltage and the current that your circuit needs. So
for a 5-volt regulator, a 12-volt, 1.5-amp supply is fine. Twelve volts is on the low side for
good stable regulation, but having 1.5 amps available makes it a good power supply for a
great many things. Each LED uses about 5-10 milliamps. Each servo uses about 20
milliamps. Each motor, 20 to 50 milliamps; and so on. (One amp is 1000 milliamps.)

A power LED is a great way to monitor your power supply. Use a standard-sized LED, in

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Adjustable 350T Regulator

series with a 460Q to 1000Q (1kQ) resistor, connected across power and ground. It will dim
and flicker a little each time something like a servo or motor starts up, and that is normal. If
the LED stays dim, then you may have something wrong. Not having enough power will do
this. Regulators will just shut down or act funny if they do not have enough input voltage for
stable operation.

Adjustable regulators look the same as any other regulator, but they are not the same. Read
your documentation. They usually have no ground pin. The center pin (2) and the heatsink
are not ground and should not be connected to ground. In the LM350T 3-amp regulator, only
R2 connects to ground. In the circuit R1 connects to Volts Out and to pin 1 (Adjust) and R2
connects to pin 1 (Adjust) and to ground. You set your output voltage by adjusting R1 and R2
until you get about 1 .25 volts across R1 . Then R2 gives you the voltage range of your
regulator with your supply. It can be fun getting this to work if your supply voltage and/or
current are too low. Look at my R1 at 754Q. R1's range is affected by how big the difference
is between your supply voltage and your regulated output voltage. See the pictures.

Regulators make great battery chargers. What is a battery charger? It is about 1 .5 times
your battery's voltage with enough current to tell the battery you are charging it. Too little
voltage or current and the battery will just think you are a load. A car battery is an example:
you only need about 16 volts, but you need to supply about 10 or more amps to be a charger
for it.

Higher voltages with low current are trickle chargers. Remote-controlled toys usually have a
battery pack of two or more cells. Each cell is 1.2 volts. Just add them up and multiply them
by 1 .5 to get your starting supply voltage for charging. Use a power supply that provides two
or three times the current of your battery pack and you have a charger for it.

Charging a battery requires monitoring the current going into the battery. When you first
hook up your charger the current will be high. When the battery reaches a full charge the
current will drop. Put a resistor of about 100 ohms between the charger's "plus" lead and the
battery's "plus" terminal, measure the voltage across it and use Ohm's law to get your
charging current: l=V/R. You can use an analogue port of a microprocessor to control your
battery charger. Many things are possible.

For the battery adapter for my Picaxe 28 project board I made the circuit diagram as simple
as possible because the 350 regulator looks like any other regulator. Do not hook the
heatsink (pin 2) to ground. Use a heatsink if you need to. You may need to readjust the pots
if you use a higher supply voltage. The 350 is a 3-amp regulator so you can use a higher

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Adjustable 350T Regulator

current supply too. These kinds of adapters make robots and things easier to play with. I left
the pots in so that I could use my adapter for other things. All of the parts are available from
Jameco.com and other sites and most at RadioShack.

Enjoy

Steve

Step 1 — Adjustable 350T Regulator

Regulators

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Voltage Regulators

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• Regulators are fun!

st generated on 2012-11-02 11:36:34 PM.

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